Why Does Asparagus Make My Pee Smell Funny?

Here’s the situation. You enjoy a wonderful dinner of grilled black bean burgers, some garlic red-skin potatoes, and a heaping pile of sauteed asparagus, green and glistening in all its glory. About a half-hour later, you find it’s time to expel some liquid from your bladder. In doing so, you notice a funny smell, and not just the regular-old smell of urea and other miscellaneous solvents within the aqueous urine.

If this has ever happened to you, you are part of about 50% of people who experience the skunky, unsettling smell of urine after eating asparagus. But don’t panic– it’s not like there’s anything wrong with you. Marcel Proust even noticed it, describing it in Swann’s Way as “the precious essence that I recognized again when, all night long following a dinner at which i had eaten [asparagus], they played, in farces as crude and poetic as a fairy play by Shakespeare, at changing my chamber pot into a jar of perfume.”

Warning! Scientific Content:

Setting Shakespeare aside for English-blogs, I’m here to explain the biological facts. When you eat asparagus and digest it, your body metabolizes a compound called methanethiol (also called methyl mercaptan) which contains sulfur, which we all know smells a little like rotten eggs.

Another culprit is thought to be asparagine, an amino acid that is known to smell badly when heated.

Depending on which scientist you choose to ask, he or she may answer that one or the other of these molecules produce the smell, while others may claim it is a combination of the two.

So… why does this happen to only 50% of people?

Believe it or not, there is a debate over asparagus. It was once thought that only half of the population can smell the funk because only half the population had the enzyme that could metabolize the odor-causing molecules. Upon investigation, though, scientists found that everyone can break down the components. Instead, the new theory suggests that only half the people have the gene that allows the nose and brain to receive and process the odor.

A fun idea for this weekend’s party:

Put theory into action! Gather up your friends and eat a pile of asparagus. Wait a half hour, then have each person pee into a cup with his or her name on it and record whether he or she can smell the funk. Also check to see if some people can smell the funk in cups where others cannot and record that. Then, send the cups off to be analyzed for the presence of methanethiol and asparagine. Use the following for some DIY science:

Hypothesis 1: Only 50% of people smell “asparagus pee” because only 50% can metabolize it.

Prediction 1: If Hypothesis 1 is true, then the cups where the individual did not smell the funk will have no methanethiol or asparagine.

Hypothesis 2: Only 50% of people smell “asparagus pee” because only 50% have the gene to recognize the smell of methanethiol or asparagine.

Prediction 2: If Hypothesis 2 is true, then the cups where the individual did not smell the funk will still have methanethiol or asparagine, and the funk will be able to be smelled by some and not by others.

It’s qualitative and not quantitative, but it’s a fun way to really get to know people.

Whatever future studies of asparagus may hold, asparagus is a very healthy food choice. Make sure you eat your vegetables…